Earlier this week I wrote about how the largest dinosaurs – 100-foot-plus titans such as the sauropod Supersaurus – got to be so mind-bogglingly huge. The post was a response to baseball player Jose Canseco’s tweets that weaker Jurassic gravity allowed such enormous creatures to exist in the distant past.

On the surface, the idea that long-necked sauropod dinosaurs had pseudo-hearts or blood accelerators seems like a fantastic sci-fi invention. But it’s an idea that paleontologists have actually entertained and addressed in their quest to understand the huge animals. There’s no evidence that dinosaurs of any sort had bizarre accessory hearts, butthe idea has still played a small role in the ongoing investigation into how giant dinosaurs actually lived. To start, we have to go back to ancient bones and the ways in which paleontologists put them together.

We all know, as the fictional fossil expert Anne Elk once described, that sauropod dinosaurs were “thin at one end; much, much thicker in the middle; and then thin again at the far end.” True enough. But how the bulky dinosaurs managed to hold up their anterior “thin end” – the neck – depends on aspects of bones, soft tissue, and biology that are still being debated by paleontologists. And, of course, sauropods were not a totally uniform group of dinosaurs that all did the same thing. Still, to get at how these dinosaurs kept blood pumping to their brains, we first have to consider how sauropods held those elegant necks in the first place.

Some researchers, such as Kent Stevens and J. Michael Parrish, have proposed that the best fit for the cervical vertebrae of sauropods such as Apatosaurus and Diplodocus creates a neck posture held almost straight out and “gently declined” from the rest of the body. In this reconstruction, the dinosaurs would have been giant Mesozoic vacuums – snarfing up ferns and other low-lying plants within the reach of their necks.

Other paleontologists disagree. In 2009, Mike Taylor, Matt Wedel, and Darren Naish made the case that sauropod dinosaurs typically held their necks aloft. The evidence not only came from the way dinosaur bones fit together, but from the anatomy and posture of living animals.

Among mammals, birds, lizards, crocodylians, and other land-dwelling vertebrates alive today, “the neck is maximally extended and the head is maximally flexed, so that the mid-cervical region is near vertical,” wrote Taylor and collaborators. The same was probably true of sauropods, too. “Unless sauropods behaved differently from all extant amniote groups,” Taylor and coauthors argued, “they must have habitually held their necks extended and their heads flexed.” The dinosaurs could have lowered their necks to graze, but, according to the study by Taylor and colleagues, the dinosaurs held their necks high as they tromped around the Mesozoic. Even though the neck posture debate isn’t over, I believe that Taylor, Wedel, Naish, and other authors have made a strong case that sauropods typically held their necks high.

Neck posture matters for estimates of blood pressure. Just think of Supersaurus, with its 50-foot neck. The sauropod would have required a considerably more powerful heart and higher blood pressure to pump blood to a head held aloft than to one that was kept lower down, closer to the height of the heart. Researchers such as Roger Seymour and Harvey Lillywhite have used this physical constraint to argue that sauropods must have maintained their necks in a low-slung posture, similar to what Stevens and Parrish proposed.

After calculating the pressure that a sauropod such as the approximately 80-foot-long Barosaurus would have required for blood to reach the dinosaur’s head (estimated at 700mmHg), Seymour and Lillywhite wrote that the left ventricle of the dinosaur’s heart alone would have weighed two tons, or about fifteen times heavier than the left ventricle of an equally-long fin whale. And even if such an enormous heart existed, the researchers reasoned, the organ would have been grossly inefficient. Such a monstrous heart seemed unlikely. Based upon their estimates and calculations, Seymour and Lillywhite concluded that sauropod dinosaurs probably had more reasonably-sized hearts and, therefore, did not have the pumping power to hold their heads high.

Could sauropods have had some kind of workaround to escape the constraints that Seymour and Lillywhite outlined? Robert Bakker thought so. In a 1978 article speculating on dinosaur feeding behavior and the origin of flowering plants, Bakker suggested that sauropods “could have used contractions of neck musculature as a relay pump to carry the cranial arterial supply.” Yet no evidence of such a feature has ever been found in sauropods or, for that matter, any vertebrate. More than that, there’s no developmental mechanism that would explain parts of blood vessels being co-opted into heart-like pumping stations, not to mention that such a series would have required a great degree of coordination between the hearts and the dinosaur’s brain. Lacking a living sauropod to study, the idea is untestable. Likewise, a proposal that big sauropods had a special “siphon” system – in which falling blood facilitated rising blood – failed to gain acceptance. So far as researchers have been able to discern, sauropods did not have accessory hearts or a special siphon system to help them circulate blood.

So what did sauropods do? Were they permanently restricted to a low-slung lifestyle of sucking down low-lying plants? Probably not. As Andreas Christian pointed out in a 2010 study of the dinosaur Euhelopus, the anatomy of some sauropods very clearly demonstrates that there were species which kept their necks habitually elevated. And as costly as this might be in energetic terms – such as fueling a powerful heart – the advantage gained by being able to feed high and low may have circumvented the drawbacks. In terms of expending energy, Christian noted, standing in one place to feed over a large vertical swath is less expensive than having to walk great distances when you’ve got a high-running metabolism to nourish. Some sauropods undoubtedly held their heads aloft, as much as 26 feet above the level of their heart in a dinosaur like Giraffatitan. The puzzle is how their bodies were capable of achieving such a wonderful feat. We need to know much more about dinosaur organs and physiology to solve the question.

Despite anatomical clues indicate that some, and perhaps most, sauropods held their heads high, the fact is that paleontologists have yet to solve the mystery of how the dinosaurs solved biological problems involving bloodflow. As Bergita Ganse and coauthors relate in a recent review of sauropod physiology, “The cardiovascular system is still a field of speculation.” Barosaurus and even larger dinosaurs probably required big hearts and exceptional blood pressures, but, lacking soft tissue structures, researchers can only approach such biological details in outline.

A giraffe bending down for a drink. Imagine a sauropod lowering its neck in the same way. Photo by Steve Garvie, image from Wikipedia.

And all of this is to say nothing of other circulation problems that come with being big. When a Supersaurus lowered its neck to feed at ground level, the dinosaur would have risked a terrible rush of blood to the head. Although very, very distantly related – seeing as they are mammals – giraffes hint at one solution for coping with this problem. The long-necked mammals have a web of little arteries called the rete mirable, Ganse and coauthors point out, which helps prevent blood pressure from skyrocketing while the giraffe’s neck is lowered. Sauropods could have evolved a similar structure. No one has yet found evidence of such an arrangement for sauropods, but it’s still a hypothesis worth entertaining since the feature exists in another long-necked vertebrate.

Then there’s the question of how sauropods prevented blood from pooling in their limbs. Once the blood went out to these extremities, how did it overcome gravity to go back to the heart? Again, mammals hold possible answers. In horses, Ganse and coauthors point out, there’s a kind of cushion that sits between the sole and the foot bones. Blood collects in the cushion, and is squeezed out into the veins when the horse steps. Giraffes have a different way of getting around the same problem. In addition to fluid pressure and details of the capillaries, the tight skin of giraffe legs helps keep blood moving through spaces where it might otherwise accumulate. We may never know how sauropods got around the same problem, but, given their size and facts of their biology, they must have had some mechanism to prevent edema. Foot cushions or tight skin are reasonable possibilities.

Sauropods are familiar dinosaurs. Apatosaurus, Brachiosaurus, and Diplodocus are some of the first dinosaurs we meet as children, and, for over a century, the awesome size of these dinosaurs has sparked our imagination. Despite our close acquaintance with Supersaurus and kin, though, there are many aspects of basic sauropod biology that still confound us. In many ways, we’re still only getting to know these giants. Paleontologists are assembling a more refined look at sauropods than has ever been possible before, yet it’s the questions – the persistent mysteries – that keeps sending scientists to pore over bone and wonder about long-lost soft tissues. We may not know as much as we would like about the sauropod cardiovascular system, but the monumental dinosaurs have certainly found their way into our hearts.

Related

9 thoughts on “Of Barosaurus and Blood Pressure”

In 1991, I was thrilled to see Discover magazine chose our announcement of Utahraptor as one of the top stories in science for that year. However, I was sorely let down when I saw the trash on the facing page. A tribute to the publication of an off the cuff theory that Barasaurus must have had multiple hearts.

This is the reference: Choy DSJ, Altman P. The cardiovascular system of Barosaurus: an educated guess. Lancet 1992; 340: 534-536. The bigger a heart is, the slower it beats. Therefore the blood would run back to the heart before it reached the brain. Because of that, there’s another theory that the Barosaurus had 8 hearts: Two in the chest and three pairs in the neck, which all worked together.
Occam’s Razor; it ain’t!

Much simpler to develop valves in the veins and arteries to control blood flow and pressure as in a giraffe.

Convincing stuff from Christian that Euhelopus could have raised its neck without causing any significant extra stress on the vertebrae and benefitted energetically at the same time. In your opinion Brian, what research needs to be done to prove how the neck was positioned at rest?

Very much appreciated the Monty Python reference as well. “This theory…”

Is the thinking too linear? Brain storming this problem, could we think of much thinner blood?
How about a design callung for much less need of blood to begin with?

Also, on the neck position, the rationale for a giraffe-like elevated stance makes sense because there is an evolutionary advantage in the higher reach. What would be thenone for a hugely long horizontal neck?

I grew up reading and hearing that dinosaurs had a second brain at the base of their tails. This was treated as common knowledge. A lot of books I read as a kid also said that larger dinosaurs (sauropods, specifically) had to live in the water because they were so bulky.

Human mammals also give us clues. Our leg muscles help pump blood back to the heart. I wouldn’t be surprised if that sort of arrangement was one of perhaps many mechanisms that evolved with long necks. All sorts of anatomic quirks may have been coopted.

Brian, does being warm-blooded or cold-blooded matter for something like blood pressure? I remember reading a theory that claimed sauropods had a more warm-blooded body chemistry in their formative years. This was considered to be an explanation of their rapid growth. Then, once they were of a comfortable and predator-proof size, their internal chemistry changed to be more like a cold-blooded animal. Is there any credence to this theory, and could it explain how the most massive adult animals could keep the blood moving?

Would it be possible for the arteries to be made of a smooth muscle like an intestine? This would allow for blood to be pushed around by the arteries themselves as a booster mechanism which would relieve stress on the heart. Smooth muscle could be triggered by the heartbeat, and it could also regulate pressure when the head was lowered. This would also be different than little pumping stations in the neck. Would this get by all the issues of having a huge body?

Like several other commenters, I thought of the valves we have in our veins – could not something similar have evolved in sauropods’ carotid arteries? With periodic contraction, the artery would function as a pump.

Furthermore, if the artery were bound to the esophagus, perhaps that periodic contraction could have been driven by the action of swallowing. I am thinking of the artery being squeezed either by an expansion of the esophagus as a food bolus passes by, or by the circular bands of muscle that contract the esophagus in peristalsis, if the artery passed through or inside those bands.

Alternatively, though less elegantly, perhaps sauropods periodically charged their heads with blood by lowering them, and maintained pressure while the head was raised by closing sphincters in the arteries and veins at the base of the neck. Given that some whales can go for an hour or more between breaths, perhaps a sauropod could go for tens of minutes between cranial blood changes?

Such sphincters could also control the increase in cranial pressure when the head was lowered – just restrict the arteries and open up the veins. I can think of a mechanism which works that way to give variable pressure within a human organ…

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Ed Yong is an award-winning British science writer. Not Exactly Rocket Science is his hub for talking about the awe-inspiring, beautiful and quirky world of science to as many people as possible.
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